The present invention relates to refractory metal nitride powders particularly those of Ta, Nb and more particularly to such powder as usable in solid electrolyte capacitors.
Solid tantalum capacitors are built on tantalum metal substrates with a dielectric layer composed of anodic tantalum oxide. A well-known problem with this structure is instability caused by oxygen migration from the oxide to the metal substrate during thermal cycling (ref. 1). A gradient of oxygen deficiency with an associated conductivity profile is created across the anodic oxide film.
The present invention focuses on capacitors using a porous mass of sintered nitrides (particularly TaN, Ta2N, NbN) as the anode and nitride powders suitable for making them. The powders are referred to as xe2x80x9csubstratesxe2x80x9d in relation to thin conversion xe2x80x9ccoatingsxe2x80x9d of dielectric oxide formed at the surfaces of the sintered powders of an anode slug (of various geometric forms) when contacted by a wet electrolyte under electro-formation conditions. In the final product the pores of the slug can be filled with a wet or solid electrolyte, but the invention is specially beneficially for solid electrolyte capacitors. Capacitors containing a conductivity profile have high bias, frequency and temperature dependence of capacitance. Although capacitor manufacturers have developed procedures to minimize or eliminate the oxygen deficiency and associated conductivity profile in the dielectric, the thermodynamic instability at the metal-metal oxide interface can contribute to ignition and other failures of solid electrolyte tantalum capacitors.
Work over the last 30 years (ref. 2 and references therein) has demonstrated that anodic oxide films grown on tantalum substrates doped with nitrogen are more stable to thermal cycling. If the nitrogen content in the tantalum exceeds 10 at, %; oxygen migration across the tantalum-tantalum oxide interface is suppressed. Capacitors made using a nitrogen doped tantalum substrate are significantly less sensitive to the effects of thermal cycling. In most cases, the substrates were thin films produced by sputtering tantalum in a nitrogen atmosphere. Sputtered substrate capacitors are found as micro devices in integrated circuits. The sputtered capacitors made using a Ta2N (33 at. % N) substrate were especially stable.
There is good potential for using TaN as the substrate for making a bilayer suitable for making a solid capacitor. The anodic film grown on this substrate is insensitive to the effects of heat treatment because the presence of nitrogen blocks the migration of oxygen across the substrate-anodic film interface. The experiments show solid capacitors made of powder with the TaN anodic film system can have negligible bias, frequency and temperature dependence of capacitance and be less susceptible to failure during long term aging. This was not made available to the art prior to the last 20 years (see Ref. 3 [copy appended] and references cited therein).
The use of nitrogen to improve the performance of tantalum capacitors made using tantalum as the substrate is known. U.S. Pat. No. 5,948,447, granted Sep. 5, 1995 to H. Chang/Cabot Corp., describes nitrogen doping (at levels of 500-7000 ppm) of tantalum or niobium powder substrates to reduce leakage and speculating a beneficial effect in limiting crystal growth during anodization and the benefit of higher solubility of nitrogen in tantalum or niobium (compared to oxygen) to lower leakage by limiting movement of oxygen and a synergistic effect of controlled combinations of nitrogen and oxygen content of the tantalum or niobium substrate. T. Tripp et al/H. C. Starck, Inc. in a symposium paper have described a 30 year long effort to investigate the effects of nitrogen doping on tantalum substrates, mostly as to sputter deposited layers of tantalum or tantalum nitride but including also nitrogen doped powder and describe current work that verifies the effect of nitrogen in retarding migration of oxygen across the metal (Ta or Nb)-anodic oxide interface. D. J. Werder et al/Bell Telephone Labs (Thin Solid Films 323 (1998): 6-9 provide transmission electron microscope images showing a tantalum pentoxide anodic film formed on a sputter deposited TaN substrate with nitrogen rich inclusions in the lower (toward the substrate) portion of the oxide layer that appear to be associated with a decrease of dielectric constant.
Most examples of this technique involve thin film nitrogen substrates prepared by sputtering tantalum in a nitrogen atmosphere. Niobium nitride powders are also disclosed for usage as substrates in electrolytic capacitors in the published PCT application WO 98/38660 (PCT/JP98/00823 filed Feb. 27, 1998 by K. Naito, Y. Uchida/Showa Denko KK), in an overall process and product system involving nitriding a niobium powder (made by chemical reduction of a niobium fluoride precursor) to produce niobium nitride powder, sintering the powder, oxidizing to form a niobium pentoxide layer (or forming such a layer in situ by a chemical vapor deposition process from an external precursor) as the dielectric, incorporating a variety of electrolytes in the pores of the sintered compact but preferably organic semiconductor systems and adding a cathode and packaging to define the complete electrolytic capacitor.
The invention also includes a niobium powder that is derived directly from a pure niobium pentoxide (Nb2O5), e.g. by reduction of the pentoxide with a magnesium vapor to form a niobium powder of extremely low oxygen impurity content and no pentoxide content at all, then introducing nitrogen in a reactor schedule that precludes re-oxidation of the niobiumxe2x80x94the schedule having multiple stages of thermal processing and environmental control defined below to establish a niobium nitride powder compound without excess of nitrogen remaining and eventually cooling under inert atmosphere and air release of the powder to form only a limited oxide at room temperature.
Another objective is a substrate that provides for a more thermodynamically stable substrate-anodic film interface making the system less stable to the degradation that occurs in the niobium-niobium oxide system, (and even in the tantalum-tantalum oxide system) during thermal cycling.